A USB mass storage is a great risk to various servers and computers. As the risk, for example, an information leakage may occur due to information extraction or a virus infection may occur due to virus mixture, on the contrary. For the risk, system measures are taken on the basis of thorough operation management. For example, use of the USB mass storage is restricted by installing device management software on each terminal or it is confirmed whether or not a virus is mixed in a file to be exchanged by installing antivirus software.
However, these software can be installed only in a corresponding operating system (OS) and the software cannot be applied to an Internet of things (IoT) device or a programmable logic controller (PLC) using an old computer or a dedicated OS. Further, in a computer used in a performance designed system such as a control system, since performance is affected by installation, it is difficult to install the software additionally.
Therefore, conventionally, a USB relay adapter type device is used to relay the device and connect the device to a USB memory, thereby executing a virus check of the file in an adapter (for example, refer to PTL 1). In PTL 1 (for example, a paragraph [0097]), “data including a computer virus program with which a computer is infected can be reliably prevented from infecting the USB memory connected to the computer.” is described.